Reaction of NO2(g) with NaCl(100)

Abstract

The heterogeneous reaction of 2NO2(g) + NaCl(100) → NaNO3(s) + ClNO(g) has been investigated using infrared spectroscopy. Single crystals, chosen for their well-defined and almost defect-free surfaces, were exposed to NO2 at four pressures: 0.15, 0.52, 1.5, and 3.5 mbar at 298 K. The formation of NO3- ions on NaCl was quantified from the photometry as the reaction progressed. The number of NO3- ions produced indicates that the reaction is not limited to the initial NaCl(100) face, but that nitrate ions also penetrate into the upper layers of the substrate. Despite an excess of NO2, the NaCl crystal transformation to NaNO3 ceases in a few hours. Gas phase ClNO is also observed spectroscopically. The strongest solid-state feature from the reaction is a well-defined sharp doublet at 1360 and 1414 cm-1 assigned to the ν3 asymmetric stretching vibration of NO3-. Both bands grow in unison as the reaction progresses. We suggest that the doublet results from the splitting of the ν3 degenerate mode in the monomers, dimers, or small ordered arrays of NO3- ions bound to neighboring ions of the NaCl substrate. There are as many as three regions that characterize the growth of NO3- at each NO2 pressure. Two of these regions are used to measure rates of NO3- production. The order of the reaction was determined from these rates and was found to be quadratic in NO2 pressure. A solid−gas reaction probability was also obtained from these reaction rates, assuming N2O4 is the reactant, and was measured to be (1.3 ± 0.3) × 10-6. This reaction probability is smaller by nearly 2 orders of magnitude than those reported by other research groups who have used powdered NaCl substrates. Probability differences are not surprising since the kinetics of this reaction is expected to depend on the concentration of defects at the surface. A mechanism, involving fracture of the NaCl substrate and diffusion of NO2 into its interior, accounts for the different growth regions and the production of NO3-. Finally when a small amount of water vapor is introduced with NO2, the production rate of NO3- is enhanced dramatically. The possible roles of H2O in heterogeneous chemistry of NaCl are discussed.